Steel product, such as slabs, blooms, bar stock and semi-furnished products are re-heated prior to hot working to produce hot rolled steel products. One conventional form of furnace used for such re-heating has been a low output top-fired furnace in which product to be heated sit on a refractory hearth; with the product conveyed through the furnace by pushers. Such furnaces have been improved to achieve higher outputs, by utilising top and bottom firing to reduce the time required for conduction of heat energy in the product to achieve a uniform temperature throughout the product.
A more recent development is the so-called walking beam furnace. This offers several advantages over the pusher furnace. The present invention principally is concerned with improvements applicable to the walking beam furnace. However, the invention can be used in at least some forms of pusher furnaces.
One benefit of the walking beam furnace is that it is self-conveying. This is achieved by having several fixed beams, extending through the furnace from the front or inlet to its exit, and several moving beams substantially parallel to the fixed beams. Product to be re-heated is supported on the fixed beams at successive locations along their length. The moving beams are actuated by hydraulic cylinders located under the heating chamber of the furnace, so as to be movable from, and back to, a lowered or ambush position so as to lift and index product forward, as required, from one to a next position on the fixed beams.
There was a major advance in walking beam furnaces in the mid-1960's when the Surface Combustion Co. of the United States of America developed a top- and bottom-fired furnace with moving beams comprising water-cooled lifting rails. This provided for supporting product alternately, and for substantially equal intervals, on the moving and stationary beams of the walking beam conveyor system. Also, by suitable design of support structure of the beams, it was possible to heat thick product slabs without the need for a soaking refractory hearth. Such hearth was used in pusher furnaces, and in early forms of walking beam furnaces, to enable regions of the lower surface of the product, cooled by contact with the support structure, to attain overall temperature uniformity throughout the product.
Over the last thirty (30) years, almost all new re-heating furnaces have been of the walking beam type. It is believed that about 50% of all furnaces for re-heating steel product are of that type, with about two hundred (200) such furnaces currently in operation throughout the world.
In the fixed and moving beams of the furnace based on the Surface Combustion Co. furnace, and in developments thereof, the water-cooled lifting rails comprising the beams have rider or skid bars mounted thereon. While designated as bars, the rider or skid bars can be of a variety of forms. Those based on the 1960's developments are in the form of buttons or cylinders about 75 mm high, 50 mm in diameter and located at about 300 mm spacing along the beam. However, other forms of rider bars in use are more rectangular in plan view, such as about 35 mm transversely, and about 140 mm longitudinally, of the beam and at a closer pitch interval so as to more closely resemble continuous rider bars. Usually, the rider bars are detachably mounted on their water-cooled pipes. This form of mounting may, for example, be by steel keeper plates which are welded to the pipe, and which are able to be ground off when replacement of rider bars is required.
The water-cooling of the pipes comprising the fixed and moving beams have a large cooling effect in the lower region of the furnace chamber. To reduce this, the cooled beams are insulated by shaped refractory insulation which encloses the beams except along the line of its rider bars. However, the insulation shields the bottom surface of the product from heating burner flames, thereby causing cold spots in the product. Also, as the product becomes hotter during its passage through the furnace, there is a further cooling effect as heat is conducted from the product to the beams, through the rider bars.
In an effort to increase furnace efficiency, there has been a trend towards operating the water-cooled pipes as part of an evaporative boiler system. In this way, steam can be generated in the pipes, and it can be used for process purposes elsewhere in the plant in which the furnace is located, or to generate electric power through a steam turbine. Even so, pipe temperatures are raised by only about 60.degree. C. above temperatures attained in water-cooled systems and, while this benefits overall energy utilisation, it has little effect on the problem of shielding of the product by pipe insulation and subsequent conductive cooling caused by the rider bars of the bottom surfaces of product being re-heated. As will be appreciated, cold spots on the product bottom surface result in imperfections in subsequent rolling of the product.
The design of the rider bars, and their attachable mounting on the pipes, can reduce the problem of conduction of heat from the product to a degree, relative to continuous rider bars integral with their pipes. Also, the spacing between rider bars relative to the distance through which the product is moved or indexed by the movable beams can, in some arrangements, vary the location at the bottom surface of the product at which heat loss by conduction occurs. However, the rider bars are of temperature resistant steel, frequently of high cost, high cobalt steels, such as with 30 to 50% cobalt. An example of a suitable metal is that available under the trade mark UMCO. The rider bars therefore inherently have a high thermal conductivity which precludes features of design and spacing from being able to significantly reduce the problem of thermal conduction.